The present invention relates to data communications and more particularly concerns a communication protocol designed to resolve clashes in a system where a plurality of separate devices (termed herein "slaves") may attempt to communicate simultaneously with another device (termed herein "master") over a common communication channel.
One field of "master and slaves" communication with which the invention is particularly concerned is a so-called "hands-free" access control system. In such a system the "slaves" comprise individual tokens worn or carried by different persons and capable of trasmitting respective identifiction codes to "master" control units associated with doorways in a building, when in the vicinity thereof, so that a control unit can determine if an approaching person is authorised to pass through without requiring the person to request access by means of a specific manipulative action. A similar system can be used for monitoring the whereabouts of persons within a building, e.g. patients within a hospital, where each wears a "slave" token the transmissions of which are received by "master" control units distributed throughout the institution and linked to a central monitoring station. Systems such as these can operate through a variety of communications media, including by way of infra-red, ultrasonic or radio emissions, or inductive or capacitative coupling techniques. In principle, however, the invention is not restricted to any particular field of application or communications medium, and may be found of general utility in resolving clashes between simultaneous slave-to-master transmissions in wireless or wired data communication environments.
Assuming that data is transmitted in binary form, the communication protocol employed in any such system must enable the simultaneous reception of a "0" and a "1" from different slave transmitters to be recognize and resolved. Furthermore, the reception of a "0" or a "1" must still be reliably achieved if a number of slaves simultaneously transmit that same value. The protocol of the present invention is based upon binary coding having the characteristics of the known Manchester II code. In Manchester II code the values "0" and "1" are distinguished not by different high and low logic levels but by the position of a high logic level in either the first half or the second half of a defined bit period. There is therefore a logic transition at the middle of each bit period, the binary value of data being represented by the direction the transition. By way of example, the accompanying FIG. 1 is a representation of the word "10110" in the Manchester II code where a "0" is represented by a high level in the first half of a bit period and a "1" is represented by a high level in the second half; (of course in any particular implementation the definitions of a "0" and a "1" could be exchanged if desired). If such a coding scheme is to be used in a system where two or more slaves transmit different data simultaneously over the same communication channel it will be appreciated that when both a "0" and a "1" are transmitted by different slaves in the same bit period the signal received by the master will be a high level throughout that period and that in the absence of some means of resolving the clash the data will be unreadable. It is to the resolution of such clashing transmissions that the present invention is directed.